Intermediate frequency stages (“IF stages”) play a key role in a number of applications which require frequency conversion, such as television tuners, set top boxes, and the like. In an exemplary IF stage, a series of signal processing components might be used to convert a signal centered at an IF frequency to a baseband or unmodulated signal suitable for direct extraction its information content.
IF stages typically require filters having frequency selective components that tend to resist integration into an integrated circuit. Other circuits commonly present in an IF stage are oscillators and phase locked loops (“PLLs”). Oscillators often require external crystals or other frequency selective components to form a resonator. PLLs often require an external loop filter. Utilizing digital signal processing techniques would tend to reduce the need for frequency selective components. By sampling an incoming signal and assigning a numerical value to the samples the signal may be processed digitally. After digital processing the numeric values may be converted back to a signal. Digital signal processing techniques tend to allow parameters, such as pass band characteristics of a filter, to be programmed and otherwise adjusted. Thus, a digital IF providing programmability to accept video signals formatted to various transmission standards can be fabricated.
Those having skill in the art would understand the desirability of having a digital IF capable of being integrated on an integrated circuit substrate that does not have the problem of requiring a large external component and that may be programmed for multi standard operation. This type of device would necessarily provide a more compact circuit, by having a tunable IF economically integrated onto an integrated circuit.